Disappointed by this second defeat, Catiline formed a conspiracy to overthrow the government and install himself in power. He acted in league with C. Manlius, who had gathered a band of Sullan veterans and other malcontents in Faesulae (in Etruria). Cicero’s warnings to the senate were at first disbelieved. But when the conspirators’ rising in Etruria was independently confirmed, he obtained the senatus consultum ultimum authorizing him as consul to act in the defense of the state. The waiting continued until, on the night of 6–7 November, assassins appointed by Catiline appeared at Cicero’s door. He had, however, been forewarned and denied them entry. That event spurred Cicero to denounce Catiline in the senate (Catilinarian 1), leading Catiline to depart Rome. Though further conspirators remained in the City, Cicero was able to obtain evidence against them and a decree of the senate calling for their execution, which he supervised.

Cicero continues his philosophical writing with a cosmological work, On the Universe, which he seems to have abandoned in favor of the dialogue On the Nature of the Gods, which set out and refuted the theologies of the Epicureans and Stoics. This was supplemented by On Divination, a dialogue between Cicero and his brother, Quintus. He also wrote the short dialogue On Old Age to honor his friend Atticus on his sixty-fifth birthday. His last quasi-forensic speech was a defense before Caesar of the Galatian tetrarch Deiotarus; the matter was still pending at Caesar’s death. He continued to attend meetings of the senate but was privately critical of the hollowing out of republican institutions under Caesar’s dictatorship. Caesar’s assassination on the Ides of March raised brief hopes of a return to republican patterns of government, but Mark Antony, who had been Caesar’s co-consul, was quickly able to reassert control.

Cicero was mostly disappointed by the news from Rome, especially the fact that the consuls Antony and Dolabella had five-year governorships voted for them in an assembly. Cicero, meanwhile, cultivated A. Hirtius, designated consul for 43, with the dedication of the work On Fate. He also wrote On Glory, with reflections on Caesar’s short-lived glory, and the Topics for his friend Trebatius. His plan to visit his son, who was studying philosophy in Athens, having been thwarted by adverse winds, he resolved to return to Rome to join the opposition. His First Philippic, delivered on September 2, politely criticizes Antony’s policies. When Antony replied with a searing attack on Cicero, the orator replied in the undelivered Second Philippic. After writing On Duties, dedicated to his son, he returned to Rome and argued in the Third Philippic for regularizing the commands of Octavian and D. Brutus in opposition to Antony, an argument summarized before the people in the Fourth Philippic.